This invention is in the field of environmental air sampling and air monitoring where an electrically powered air pump is required. The accuracy and reliability of the air sampling pump is a major consideration since it affects the accuracy of the air contaminant measurement. Pumps of this type need to be made small and battery operated for portability so that they may be used in applications such as personal air exposure monitoring. Air sampling pumps are commonly used to draw air through collecting devices, which are designed to trap the contaminants in the air within the device and store same for later analysis. When analyzing the contaminants trapped in the collector device, the total volume of air that passed through the device during monitoring must be known. The average flow rate of the air through the device during monitoring must also be constant over the monitoring period.
There are two basic types of air sampling pumps. One which is a high flow rate pump in the range of one (1) to three (3) liters per minute which is used for collection of particulate matter in air. The other type pump is a low flow rate pump in the range of 1 to 300 ml./minute which is used for the collection of chemical compounds present in the air.
Most standards today, for the measurement of personnel exposure to hazardous/toxic vapors, are specified in terms of time weighted average (TWA). The TWA exposure to a compound is the average concentration of that compound over a specified period of elapsed time as measured in the breathing zone of an individual. In the industrial hygiene field, the TWA exposure is the common parameter and is normally specified for a period of 8 hours. Additional specifications, such as average exposure over a fifteen minute maximum period and absolute peak levels, are also sometimes specified. The 8 hour TWA is related to an employee's exposure during a normal work day and is considered to be the most meaningful method of assessing his exposure to a potentially hazardous vapor environment.
Peak exposures must be measured with a continuous measuring instrument. Fifteen minute TWA exposures can be measured by either averaging continuous readings or with an integrating method such as a sample collection bag or sample collecting tube. Accurate measurement of the 8 hour TWA is most readily accomplished using a full-period (continuous) sampling through a collector tube, wherein the compounds in the air are trapped in a sorbent material. The sorbent material is contained in a tube through which the air is drawn at a constant rate during the full 8 hour period of concern. Periodic sampling, to arrive at an estimated eight hour TWA, can also be used; however, the results must be factored to arrive at a statistical lower confidence level (LCL) which must be reported with the data.
In recent years, many portable air sampling pumps have been introduced for pumping air through collector tubes as a means of obtaining samples for the determination of TWA exposure. As the methodology has developed, it has become increasingly apparent that the performance of the sample pump is very critical to the accuracy of the air contaminant measurement.
Sampling pumps for time-weighted average (TWA) measurements must maintain a constant flow rate, even if provisions are made for determing the total volume pumped. Any pump can have a flow rate malfunction, due to changes in flow restriction as well as other factors, i.e., every air pump malfunctions when the input line is blocked. The possibility of a fault cannot be eliminated.
An error in the value of "total volume pumped" produces a like error in the time-weighted average (TWA) measurement variations in sample flow rate produces less pronounced error in the TWA measurement but cannot be compensated for without knowledge of flow rate and contamination level as a function of time. If no independent means is provided for measuring the total volume pumped, this value is calculated from flow rate and becomes dependent on the flow rate accuracy. The total operating time of a pump must be known to calculate total volume from flow rate, i.e., total volume = time X flow rate. If a pump stops or is turned off for an unknown period during monitoring, the total volume will be unknown.
There are many air sampling pumps on the market for use in drawing air samples through chemical trapping collector tubes. These existing pumps use a variety of methods for accomplishing their task of pumping air at a constant average rate over a period of time in a fashion such that the total volume of air through the collection device is known at the end of the air monitoring period. The most common type pump is a standard motor driven diaphragm pump which operates through a gear reduction train such that the speed of the diaphragm pump can be varied to establish the flow rate of air through a given air collection device. In application the restriction to air flow and resulting pressure drop across a collector tube can vary as function of time, as can the speed of the motor driven diaphragm pump due to variations in the supply voltage to the motor. These type variations can cause variations in the air flow rate through the collection device which would make it impossible to arrive at an accurate measurement of total volume of air through the collector during the monitoring period.
Pumps are available which incorporate an air flow sensor which is used to sense the rate of air flow through the pump and servo a signal back to the pump motor to control its speed such that it maintains a constant flow rate during the monitoring period. Other type air sampling pumps measure the total volume of air pumped through the collection device by counting the strokes of a diaphragm or bellows type pump which simulates a measurement of positive air displacement. Of the many pumps available today, none are capable of providing a constant average flow rate with varying restrictions in the pumping line, while also independently measuring the volume of air displaced through the pump as related to ambient pressure air, while also providing a register of total pump operating time, and also providing a means for audible or visual alarm in the event the flow rate changes outside its specified limits.